When particulate matter is fed into a receptacle, it is frequently desirable to utilize conduits which permit gravitational flow into the crucible, protect the particulate matter from contamination by the environment, and are preferably formed of materials such that at least their inner surfaces are non-contaminating to the particulate matter. Such conduits have been used, e.g., in feeding particulate polysilicon into a melting crucible in the Czochralski method of manufacturing monocrystalline silicon from molten silicon--the preferred non-contaminating materials for these conduits usually being silicon, quartz, silicon-on-quartz, or silicon-on-graphite.
A major disadvantage of known devices for feeding particulate matter into receptacles is usually apt to be their permitting gravitationally-flowing particles to leave the conduits at considerable speed. This is likely to cause serious problems in processes such as the feeding of particulate polysilicon into a melting crucible, both when the crucible already contains a molten mass and when it does not.
When particulate polysilicon is added at normal falling velocity to a crucible containing a molten mass, there is a danger of splashing or excessive production of ripples or eddies within the mass; and ripple formation in the subsequently-formed molten mass can also be caused by the fast addition of the particulate matter to a crucible which does not already contain a melt at the time of the addition. This can happen because some of the particles, when striking one another or the crucible, can bounce up, become situated above the basin of the crucible, and later fall into the molten silicon--creating a tippling effect which disrupts formation of the desired silicon crystal.
Other problems that can be caused by the scattering of particles are that (1) the scattered particles may accumulate contaminants, such as silicon monoxide, which they will bring with them to the molten mass of silicon and (2) the scattering of the particles may cause particulate matter to locate in other areas of the silicon melting/crystal pulling apparatus used in a Czochralski system and damage or block the functioning of the apparatus.
As disclosed in U.S. Pat. No. 5,080,873 (Ono et al.), various feed tubes have been designed to permit feeding particulate polysilicon into a melting crucible at a controlled rate. These feed tubes provide advantages over the conduits which include no means of reducing the velocity of the falling particles, but they do not constitute a really satisfactory means of accomplishing the feed. Not only do they allow the initial flow of particles to exit at excessive velocities, but their static-position design necessitates their being positioned near the edge of the crucible to prevent them from obstructing crystal growth at the center. Moreover:
(1) several of these feed tubes could also allow particles to become lodged therein (especially when the flow path into the crucible is not vertical)--creating a source of particles to become dislodged and fall into or around the crucible during crystal growth, and PA1 (2) the positioning of the tubes to encourage preheating of the particles therein would create another potential hazard to the crystal growth environment if the feeding and heating were interrupted, since the preheating could cause polysilicon granules to fuse, adhere to the inner surfaces of the tubes until cooled, and then break away and fall into the crucible. PA1 (a) permit feeding the particles at the optimum central location for recharge of the crucible without providing an obstruction for crystal growth at that location, PA1 (b) minimize the particle preheating and fusion that can be caused by extension of the feed tube into the crucible, and PA1 (c) thus avoid the environmental contamination that would be caused if fused particles were allowed to form and then break away during withdrawal of the tube.
Other means of feeding particulate polysilicon into a melting crucible are known, of course, including the apparatus of U.S. Pat. No. 5,152,433 (Mohri). However, these means also fail to provide a really satisfactory way of accomplishing the feed, both because of not exerting sufficient control of the flow rate and because of having the distal portion of the feed tube in a fixed position that prevents its being easily withdrawn for repair or for allowing unobstructed crystal growth at the center of the crucible if it were located so as to permit feeding the particles into the optimum location for recharge of the crucible, i.e., near the center.
It would be desirable to find a means of controlling the velocity of the flow of particles exiting a feed tube in a manner such that (1) the flow rate would be reduced both initially and subsequently during the feed, (2) particles would not be allowed to become trapped in the feeding means, and (3) the feed tube could be lowered into a melting crucible or the like and then withdrawn so as to:
Such a means would permit avoiding the aforementioned problems associated with known means of feeding particulate polysilicon into a melting crucible, as well as similar problems encountered in known methods of feeding other particulate matter into heated receptacles; and it would provide an apparatus suitable for use in discontinuous feeding, i.e., recharge operations, rather than the continuous feeding for which the Ono et al. and Mohri feeding means are designed.